pubs.acs.org/JPCL
Artificially Designed Membranes Using Phosphonated Multiwall Carbon Nanotube-Polybenzimidazole Composites for Polymer Electrolyte Fuel Cells Ramaiyan Kannan, Pradnya P. Aher, Thangavelu Palaniselvam, Sreekumar Kurungot, Ulhas K. Kharul, and Vijayamohanan K. Pillai* Physical Chemistry Division and Polymer Science and Engineering, National Chemical Laboratory, Pune, India 411008
ABSTRACT The ability of phosphonated carbon nanotubes to offer an unprecedented approach to tune both proton conductivity and mechanical stability of hybrid polymer electrolytes based on the polybenzimidazole membrane is demonstrated for fuel cell applications. The covalent attachment between the amino group of the 2-aminoethylphosphonic acid precursor and CNTs has been confirmed by NMR and IR experiments, while EDAX analysis indicates that one out of every 20 carbon atoms in the CNT is functionalized. Proton conductivity of the composite membrane shows a remarkable 50% improvement in performance, while a maximum power density of 780 and 600 mW cm-2 is obtained for the composite and pristine membranes, respectively. Finally, the ultimate strength determined for the composite and pristine membranes is 100 and 65 MPa, respectively, demonstrating the superiority of the composite. This study opens up a new strategy to systematically tune the properties of polymer electrolytes for special applications by using appropriately functionalized CNTs. SECTION Energy Conversion and Storage
O
enhancement of proton conductivity and mechanical strength along with increased resistance toward acid leaching is a challenge yet to be overcome successfully, and a tailor-made hybrid composite with desired properties should alleviate many of these issues. Herein, we report such a successful accomplishment through the preparation of novel phosphonic-acid-functionalized carbon nanotubes (p-CNTs) and their composite with PA-doped PBI membranes (PBpNT). Functionalized carbon nanotubes with tailored ionic sites can overcome the above-mentioned issues as they provide a means to increase the ionic strength or functional group intensity coupled with mechanical robustness.11,12 The reinforcement provided by the interpenetrated network of phosphonate CNTs could possibly prevent the leaching of PA during sustained operation. A similar strategy has been recently demonstrated on Nafion-based composites with sulfonic acid (SA)-functionalized CNTs, where the hydrophilic domain size of Nafion could be varied systematically from 40 to 70 Å. This motivated us to functionalize the sidewalls of CNTs with phosphonic acid, and we accordingly adapted a dual functionalization strategy to accomplish the preparation of p-CNT (Scheme 1a, product 2). Microwave treatment (MWT) is critical as SA groups help in improving the solubility that
ne of the major obstacles in the way of making cheaper and more efficient polymer electrolyte membrane fuel cells (PEMFCs) is the inability of perfluoro sulfonic acid based membranes to work above 80 °C. However, working at higher temperatures (like 150 °C) is extremely critical to prevent CO poisoning of the Pt electrocatalyst (either present inadvertently in the hydrogen feed from a reformer or generated as an intermediate during the oxidation of fuels like methanol and ethanol) apart from the improved kinetics and higher efficiency.1-3 Accordingly, many polymer and composite electrolytes with higher proton conductivity and improved thermal stability have been sought worldwide during the last two decades.4-6 A few have even been reported recently that work above 150 °C, although their chemical stability is questionable, leading to serious durability issues.7,8 Among them, the phosphoric acid (PA)-doped polybenzimidazole (PBI) membrane is one of the most attractive choices mainly due to the benefits such as increased CO tolerance, higher reaction kinetics, zero water electro-osmotic drag, lesser balance of plant components, and better combined heat and power (CHP) generation that would boost the efficiency further.9 Nevertheless, it is the relatively lower proton conductivity, acid leaching tendency, and inadequate mechanical stability under fuel cell operating conditions that restrict their use as an electrolyte for PEMFC. Further doping of PA would increase the proton conductivity but at the expense of mechanical stability, and this is not suitable for membrane electrode assembly (MEA) fabrication.10 Hence, simultaneous
r 2010 American Chemical Society
Received Date: May 25, 2010 Accepted Date: June 17, 2010 Published on Web Date: June 22, 2010
2109
DOI: 10.1021/jz1007005 |J. Phys. Chem. Lett. 2010, 1, 2109–2113
pubs.acs.org/JPCL
Scheme 1. (a) Preparation of Phosphonated CNTs through Normal and Dual Functionalization Proceduresa and (b) Distribution of p-CNT in the PBI Matrix That Would Help Form a Network for Proton Transportb
a The two-step phosphonation resulted in p-CNTs insoluble in water, while the three step phosphonation coupled with sulfonation resulted in watersoluble p-CNTs. b The light-blue-colored domains represent the PA chemically bonded in the PBI matrix, while the pale-green-colored domains represent the PA on the p-CNTsidewalls and tips. The scheme clearly reveals network formation after incorporating p-CNT in the membrane matrix, thus preventing phosphoric acid leaching.
results in homogeneous membrane fabrication (TEM image; Figure S1, Supporting Information), while phosphonic acid groups help in increasing the functional group intensity, thus assisting a network formation for proton conductivity (Scheme 1b). Our earlier experiments without MWT resulted in p-CNTs with poor solubility that could not meet the stringent needs of membrane fabrication (Scheme 1a, product 1). Energy dispersive X-ray analysis (EDAX) measurements indicate a 15.6 wt % phosphonate group and 8.2 wt % sulfonate group on the sidewalls of p-CNT (Figure S2, Supporting Information). Approximately 1 out of every 20 carbon atoms is functionalized with either phosphonic acid or SA groups, resulting in the improved solubility in polar solvents. Further, the possibility of electronic conductivity that could arise from CNTs is greatly reduced by functionalization and a low level of CNT incorporation (1 wt %) in the composite membrane. For example, the electronic conductivity for the pristine and functionalized CNTs is 0.62 and 0.46 S cm-1, respectively, while the composite membranes with these CNTs show no appreciable electronic conductivity (>10 MΩ).
r 2010 American Chemical Society
The nature of bonding between CNT and AEP is critical, despite proving their presence, since AEP can bind to acidfunctionalized CNTs either through amide linkage or through the phosphonate group, and the latter may not be effective in improving the proton conductivity. The nature of bonding is confirmed by 1H and 31P NMR studies, where 1H NMR of AEP shows two peaks for the methylene protons at 2.9 and 1.8 ppm that are consistent with its chemical structure. After attachment with the CNT, the peak near 3 gets an upfield shift at 3.1, suggesting the formation of an amide link. Further, 31P NMR also confirms this as the peak position remains the same, suggesting that the -NH2 group is bonded to the carboxylic groups of CNTs while the phosphonate group remains free on the chain end (Figure 1a). Further confirmation arrives from the FT-IR (Figure S3, Supporting Information) of phosphonated CNTs and carboxylated CNTs (c-CNTs), where the CdO stretching peak of c-CNTat 1705 cm-1 gets shifted to 1640 cm-1 in the p-CNT, corresponding to an amide link which further confirms the covalent nature of attachment of an amine group onto the CNT surface.
2110
DOI: 10.1021/jz1007005 |J. Phys. Chem. Lett. 2010, 1, 2109–2113
pubs.acs.org/JPCL
Figure 2. (a) Polarization plots of PBI iso, PBpNT, and PBNT composite membranes measured at 140 °C by passing dry H2 and O2 at a flow rate of 0.2 slpm. The cells were conditioned at 0.6 V for 30 min. (b) Stress-strain curve for the pristine PBI and PBpNT composite membrane.
Figure 1. (a) 1H and 31P NMR of AEP and p-CNT measured using D2O as the solvent. (b) Arrhenius plot for the proton conduction of PBI iso and PBpNT membranes from 25 to 160 °C through twoprobe impedance measurements.
of 13.6 and 9.8 kJ mol-1 (calculated by linear curve fitting) for pristine and composite membranes, respectively, suggests that proton transport is much more facile in the latter. This could be attributed to the formation of a proton conducting network by the localized phosphonic acid groups on the carbon nanotubes, as suggested by Scheme 1b, where the chemically bonded PA domains of the PBI matrix and that on the CNT surface organize to provide the network. However, undoped composite membranes do not show any significant proton conductivity (
